Welcome to DC mcbain! According to the Breville grill manuals ... for a boneless chicken breast - about 6 minutes (or until done).
This is the page for the Breville manuals
- just click on the "grills" in the left hand column and pick the one you have - then when you click on it - you can find the link to the manual on the upper right edge above the photo. The manuals will give you basic cooking times for different things.
Hope this helps.